[gumby]

> Have previous protocols (TCP, UDP, etc) been in widespread use before their respective RFCs?

In general, yes: modern RFCs should address existing practice.

The very early RFCs were just that: requests for comments, i.e. memos. Sometimes reflection on existing practice, sometimes design discussions.

For your specific examples (TCP and UDP): they were part of a redesign of the Internet protocols (well a redesign of the ARPANET's protocols TBH, NCP, with the intent of enabling smooth internetworking). So those RFCs reflected the specification of new protocols, though some experimentation had been done.

I remember the transition day. The first machines, IIRC were ITS machines that some had hoped would not make the transition at all! I don't believe there were any Unix machines on the net that day -- Berkeley sockets hadn't yet been released.